A friction-roller-type speed changer generates less noise than a gear-type speed changer, such as one which uses planetary gears, even when operating at high speeds. The use of this kind of friction-roller-type speed changer in a bicycle which uses auxiliary power from an electric motor to reduce the amount of force required for stepping on the pedals has been disclosed in Japanese Patent Publication Tokukai Hei No. 7-95744.
FIG. 1 is a block diagram of an auxiliary-powered drive apparatus for a bicycle etc. To drive the load 1, such as gears or the like of the bicycle, a first input unit 2, which is the input of human power from the pedals, etc., and a second input unit 3, which is from an electric motor, are arranged in parallel. The second input unit 3 has higher speed but lower torque when compared with the first input unit 2, and in the stage that follows it, there is a decelerator 4, which reduces the speed and increases the torque of the power that is input from the second input unit 3. The input unit 3 uses a sensor (not shown in the figure) to detect tile drive force that is applied from the first input unit and to generate a drive force that corresponds to that drive force, so that even if the force applied to the first input unit 2 is small, it is possible to drive the load 1. In other words, a driving torque T.sub.2, that corresponds to the driving torque T.sub.1 generated by the first input unit 2, is generated by the second input unit 3 and decelerator 4. Moreover, both of these driving torques, T.sub.1 and T.sub.2, are combined in a combination unit 5, and the total torque T.sub.3 (if losses due to friction are ignored, T.sub.3 =T.sub.1 +T.sub.2) drives the load 1.
In the example of an auxiliarily powered bycicle, when forcibly pressing the pedal regardless of whether going down a gradual slope or driving with a strong following wind, the torque T.sub.1 applied to the first input unit 2 may be larger than the total torque T.sub.3 required for driving the load (T.sub.1 &gt;T.sub.3). In this case, of the torque T.sub.1 that is applied to the first input unit 2, the amount (T.sub.1 -T.sub.3) that exceeds the torque T.sub.3 required to drive the load 1 is transmitted back to the second input unit 3 from the combination unit 5 by way of the decelerator 4 as shown in FIG. 2 This then rotates the rotor of the electric motor which makes up the second input unit 3. As a result, the driving torque T.sub.1 that is applied to the first input unit 2 is not efficiently used for driving the load 1, and the force required to generate the driving torque T.sub.1 by the first input unit 2 (for example the force required to press the pedal) becomes uselessly larger.
Conventionally, to solve this kind of problem, a single-direction clutch is placed between the decelerator 4 and the combination unit 5, so that power is transmitted only in the direction of the combination unit 5 from the decelerator 4. On the other hand, by changing the decelerator 4 from a normal fiction-roller type to a wedge-roller type, it is possible to omit the single-direction clutch as disclosed in Japanese Patent Publication Tokukai Hei No. 9-061329. FIG. 3 shows a construction of this kind of friction-roller speed changer -of the wedge-roller type.
This friction-roller speed changer of the wedge-roller type comprises a center roller 7, whose outer surface is a first cylindrical surface 6, aid an outer ring 9 whose inner surface is a second cylindrical surface 8, and which is located around the center roller 7 to rotate freely with respect to the center roller 7. The center roller 7 is fixed to one end of a first rotating shaft so that it is concentric with the first rotating shaft, and the end of a second rotating shaft is coupled with and fixed to the outer ring 9 so that it is concentric with the outer ring 9.
In the annular space 10 between the first cylindrical surface 6 and the second cylindrical surface 8, there are three shafts 11a, 11b which are located so as to be parallel with the center roller 7 and outer ring 9, and these shafts 11a, 11b rotatably support intermediate rollers 12a, 12b, 12c. The outer peripheral surface of each of these intermediate rollers 12a, 12b, 12c is a third cylindrical surface 13 where each third cylindrical surface 13 comes in contact with the first and second cylindrical surfaces 6 and 8. Moreover, by making the center of the center roller 7 and the center of the outer ring 9 eccentric with each other, the width of the annular space 10 is uneven in the circumferential direction.
Of the three intermediate rollers, 12a, 12b, 12c, the intermediate roller 12a is a wedge roller that is supported so that it freely move a little hi the circumferential direction in the annular space 10, and through the use of a spring 14, that is pressure or biasing means, the wedge-roller or intermediate roller 12a is elastically pressed in the direction to the narrow width portion of the annular space 10.
When transmitting a rotational force using a friction-roller-type speed changer that is constructed as described above, if the center roller 7 is rotated in the clockwise direction as indicated by the arrow ".alpha." in FIG. 3, the wedge roller or intermediate roller 12a rotates in the counterclockwise direction as indicated by the arrow ".beta." with shaft 11a as the center, and the outer ring 9 also rotates in the counterclockwise direction as indicated by the arrow ".gamma.".
Here, the intermediate roller 12a rotates as shown by the arrow ".beta.", and both the center roller 7 and the outer ring 9, which hold the intermediate roller 12a therebetween, rotate in opposite directions, ".alpha." and ".gamma.", and as a result the entire intermediate roller 12a has a tendency to move in the clockwise direction of FIG. 3 as indicated by the arrow ".gamma.". In other words, the intermediate roller 12a receives a force in the direction of arrow ".gamma." from the center roller 7 which rotates in the direction of arrow ".alpha.", and the intermediate roller 12a per se rotates in the direction of arrow ".beta.", and by so doing, the intermediate roller 12a receives a force in the direction for arrow ".gamma." from the reaction received from the point of contact with the second cylindrical surface 8 that is formed on the inner peripheral surface of the outer ring 9.
As a result, as the center roller 7 rotates, the intermediate roller 12a tends to move toward the narrow width area of the annular space 10. Also, the third cylindrical surface 13 that is formed on the outer peripheral silence of this intermediate roller 12a strongly presses against the first cylindrical surface 6 which is formed around the outer peripheral surface of the center roller 7, and the second cylindrical surface 8 which is formed around the inner peripheral surface of the outer ring 9. As a result, the contact pressure at the radially inner contact point 15, where the third cylindrical surface 13 comes in contact with the first cylindrical surface 6, and at the radially outer contact point 16, where the third cylindrical surface 13 comes in contact with the second cylindrical surface 8, becomes greater.
As the contact pressure at both the inner and outer contact points 15 and 16 of the wedge roller or intermediate roller 12a becomes greater, at least one of the center roller 7 and outer ring 9, that are respectively pressed by the third cylindrical surface formed around the outer peripheral surface of the intermediate roller 12a, is displaced a little, due to an installation gap or to elastic deformation, in the respective radial direction.
As a result, the contact pressure becomes higher at two radially inner contact points 15 where the third cylindrical surfaces 13 that are formed around the outer peripheral surfaces of the remaining intermediate rollers 12b, 12c come in contact with the first cylindrical surface 6, and at two radially outer contact points 16 where these third cylindrical surfaces 13 come in contact with the second cylindrical surface 8.
The force which moves the intermediate roller 12a, which functions as a wedge roller, in the direction of the narrow width area of the annular space 10 varies according to the size of the torque that is transmitted from the center roller 7 to the outer ring 9. Moreover, as this force becomes large, the contact pressure at the radially inner and outer contact points 15 and 16 becomes greater. Therefore, the transmission efficiency of the fiction-roller-type speed changer is maintained by automatically selecting a contact pressure that corresponds to the transmission torque.
The above was an example of using the friction-roller-type speed changer as a decelerator, where the center roller 7 was taken to be the input side and the outer ring 9 was taken to be the output side. Conversely, if the friction-roller-type speed changer is used as an accelerator by taking the outer ring 9 to be the input side and the center roller 7 to be the output side, except that the direction of rotation is opposite, the other action is the same, and it is possible to transmit power between the outer ring 9 and the center roller 7, while at the same time maintaining the transmission efficiency of the friction-roller-speed changer by automatically selecting a contact pressure that corresponds to the transmitted torque.
If the members on the output side rotate at a higher speed than the speed which corresponds to the members on the input side, the intermediate roller 12a which functions as a wedge roller, tends to move in the direction to the wide portion in tile annular space 10, and the contact pressure at the radially inner contact points 15 and radially outer contact points 16 is lost, and transmission of power between the center roller 7 and the outer ring 9 is broken.
In other words, when the friction-roller speed changer is used as a decelerator, if the outer ring 9 rotates in the direction of arrow ".gamma." in FIG. 3 while the center roller 7 is stopped, the intermediate roller 12a tends to move in the direction to the wide area in the annular space 10 against the elastic force of the spring 14. When the friction-roller speed changer is used as an accelerator as well, if the center roller 7 rotates in the direction opposite to the arrow ".alpha." in FIG. 3 while the outer ring 9 is stopped, the intermediate roller 12a tends to move in the direction to the wide area in the annular space 10 against the elastic force of the spring 14.
In the case of a wedge-roller type friction-roller speed changer when the output member rotates at a higher speed than the speed that corresponds to the input member, power transmission between the center roller 7 and outer ring 9 is broken off. Therefore, in the drive system shown in FIGS. 1 and 2, even if the single-direction clutch between the decelerator 4 and the combination unit 5 is omitted, it is possible to prevent the drive force that is applied to the first input unit 2 due to the existence of the electric motor or second input unit 3 from becoming uselessly large.
As in the case of an auxiliary-powered bicycle, if the construction is such that the direction of the driving force applied to the load 1 is set, then by using the friction-roller speed changer of the wedge-roller type as shown in FIG. 3 as a decelerator 4, it is possible to both reduce cost by omitting the single-direction clutch and to maintain the transmission efficiency by optimizing the contact pressure. On the other hand, if the direction of the driving force to be applied to the load 1 is not set, then the friction-roller speed changer as shown in FIG. 3 cannot be used. In other words, with the friction-roller speed changer shown in FIG. 3, if the direction of rotation of the transmitted power is reversed, then the intermediate roller 12a, which functions as a wedge roller, tends to move in the direction to the wide area in the annular space 10, and the contact pressure at the radially inner contact points 15 and the radially outer contact points 16 is lost, and power is not transmitted between the center roller 7 and the outer ring 9. For example, the friction-roller speed changer of FIG. 3 cannot be used in a device that is driven by stepping on a pedal, such as in an amusement park ride or a pedal boat, or where it is possible to turn the pedals in both directions.
In this kind of situation, a friction-roller speed changer is used that is constructed as shown in FIG. 4, where there are three intermediate rollers 12a, 12b and 12c, of which the intermediate rollers 12a and 12b function as wedge rollers. FIG. 4 shows the friction-roller speed changer that is disclosed on U.S. Pat. No. 4,709,589. In this second example of a friction-roller speed changer, two intermediate rollers 12a, 12b of the three intermediate rollers 12a, 12b, 12c are supported so that they can each move a little in the circumferential direction of the annular space 10, and act as wedge rollers.
Moreover, these two intermediate rollers 12a, 12b, which act as wedge rollers, are elastically pressed toward the narrow width area of the annular space 10 by springs 14, respectively, in substantially opposite circumferential directions (move toward each other). With the construction of this second example, regardless of the direction of the relative rotation of the center roller 7 and outer ring 9, one of the two intermediate rollers 12a, 12b, which act as wedge rollers, is wedged into the narrow width area of the annular space 10, and maintains the contact pressure at the radially inner contact points 15 and radially outer contact points 16. Therefore, regardless of the direction of rotation of the transmitted power, it is possible to maintain the transmission efficiency by optimizing the contact pressure.
In the case of a friction-roller speed changer, as shown in FIG. 4, that is capable of maintaining the transmission efficiency regardless of the direction of rotation of the transmitted power, if the driving torque that T.sub.1 that is applied by the first input unit 2 is greater than the torque T.sub.3 that is required to drive the load 1 (T.sub.1 &gt;T.sub.3) as shown in FIG. 4 as described above, then the force required to generate the driving torque T.sub.1 at the first input unit 2 becomes uselessly greater. In other words, in the case of the friction-roller speed changer shown in FIG. 4, regardless of the relative direction of rotation between the center roller 7 and the outer ring 9, power is constantly transmitted between the center roller 7 and the outer ring 9. Accordingly, if the driving torque T.sub.1 that is applied at the first input unit 2 is greater than the torque T.sub.3 required to drive the load 1, the electric motor, which makes up the second input unit 3, in addition to the load 1 must be driven by the power applied from the first input unit 2 through human power. As a result, the force that must be applied at the first input unit 2 becomes uselessly larger, and this is undesirable.
In the friction-roller speed changer having a single intermediate roller 12a as the wedge roller as shown in FIG. 3, when the rotational force is added in a reversed direction from the side of load 1, it is impossible to separate the load 1 from the second input unit 3 (FIGS. 1 and 2). For example, when moving back the auxiliarily powered bicycle, the outer ring 9 (FIG. 3), that is the decelerator 4, tends to be rotated in a clockwise direction in FIG. 3 (opposite direction to the arrow .gamma. in FIG. 3) by the rear wheel l of the bicycle, that is load, by means of a chain and crank shaft.
In this case, the intermediate roller 12a tends to move toward the narrower area of the annular space 10 to transmit the rotational force to the center roller 7 from the outer ring 9. Consequently, when moving back the auxiliarily powered bicycle, the electric motor, that is the second input unit 3, must be rotated, and by that amount, the force required to move back the bicycle is larger.
U.S. Pat. No. 4,481,842 discloses a structure to shut off the power transmission when the torque that is to be transmitted through the friction-roller speed changer is at a predetermined value or more.
However, in this structure, if the torque is below the predetermined valve, the power is transmitted even when it is not desired. On the contrary, when the torque is the predetermined value or more, the power transmission is shut off even when the power transmission is required. Therefore, the structure does not make sense in solving the problems as mentioned above.